Polybutene is a polymer produced from a C4 olefin having 4 carbon atoms formed in the cracking process of hydrocarbons in the presence of a Friedel-Craft type catalyst. The number average molecular weight (Mn) of polybutene is about 300 to 5,000. The feedstock residue remaining after extraction of 1,3-butadiene from C-4 hydrocarbons produced in petroleum refineries or naphtha cracking centers (NCCs) that involve cracking crude oils contains paraffins such as iso-butane or n-butane and olefins, such as 1-butene, 2-butene, isobutene, etc. The isobutene content of the feedstock is approximately 20 to 50 wt. %. The isobutene is mainly used in the preparation of methyl-t-butyl ether (MTBE) used as an octane enhancer, or polybutenes. As the isobutene is the most reactive one of the olefins, the polybutenes are produced mostly from the isobutene units. Conventionally, polybutenes have been used in gluing agents, adhesives or isolating oils, and those with low reactivity are preferred. Such low-reactivity polyisobutylene are called “conventional PIB”. In recent years, polybutenes with polar groups are increasingly used in anti-scuff agents for engine oil, viscosity index improvers, or cleansers used in combination with the fuel in the internal combustion engines for automobile or the like. Such highly reactive polyisobutylenes are called “highly reactive polyisobutylene (HR-PIB)”.
The most popular one of the products obtained by introducing polar groups to the polybutene is polyisobutylene succinic anhydride (PIBSA) that is prepared by the reaction of polybutene with maleic anhydride. From the PIBSA, a variety of lubricant additives or fuel cleansers are prepared. In the preparation of PIBSA, as the double bond of the polybutene is located at the furthermost end of the polybutene, that is, the polybutene is the highly reactive polyisobutylene (HR-PIB), the polybutene can react directly with the maleic anhydride to form PIBSA with high yield. On the contrary, when the polybutene is a conventional PIB that has a relatively low reactivity due to its double bond positioned inside and many alkyl groups included as substituents causing steric hindrance, it is necessary to chlorinate the polybutene with chlorine gas and put it in the reaction with maleic anhydride to produce PIBSA.
In order to enhance the reactivity of polybutene, the polymerization conditions for polybutene are so controlled as to have the double bond of the polybutene position at the furthermost end of the polybutene as possible. The double bond positioned at the terminal end of polybutene is called “vinylidene”. The compound having a vinylidene content of 70% or higher is “highly reactive polyisobutylene”, the compound having a vinylidene content of about 40 to 70% is “mid vinylidene polyisobutylene (MV-PIB), and the compound having a vinylidene content of 3 to 40% is a conventional polyisobutylene. The choice of catalysts and cocatalysts is of great importance in the control of the reactivity of the polybutene. Generally, the catalyst is boron trifluoride (BF3) and the cocatalyst is alcohols, ethers, or the like. Further, in the synthesis of a polybutene in which the position of the double bond is not induced to the terminal, aluminum trichloride (AlCl3) is available as a catalyst to obtain a conventional polyisobutylene having a vinylidene content of 3 to 40%. In the preparation of polybutene, the n-butene included in the feedstock possibly causes deterioration in the product quality, the productivity per unit catalyst and the productivity per unit feedstock, but the higher isobutene content of the feedstock leads to an increase in the product quality, the productivity per unit catalyst and the productivity per unit feedstock.
It is particularly desirable to use a high-quality isobutene feedstock removed of n-butene in order to produce a highly reactive polyisobutylene having a high terminal vinylidene content and reduce the fluorine content in the product derived from the catalyst. Even for the production of a conventional polyisobutylene, a high-quality isobutene feedstock removed of n-butene is preferably used in order to lower the chlorine content in the product and enhance the productivity per unit feedstock or unit catalyst. There are known various methods for eliminating 1-butene that most adversely affects the quality of the polybutene among the n-butenes. For example, U.S. Pat. No. 5,674,955 discloses a method of producing polybutenes from a feedstock comprising at least 5 wt. % of 1-butene, which method is characterized in that prior to polymerization, the feedstock is subjected to a pre-treatment step in order to reduce the 1-butene content by at least 20 wt. % and then to a polymerization step using a halogen compound as a catalyst to produce a polybutene with a high vinylidene content and a low halogen content. In this method, however, the isomerized 2-butene may still cause deterioration in the catalytic activity and the mileage of catalyst. U.S. Pat. No. 6,207,115 describes a production of propylenes that includes performing selective hydrogenation of diolefin (Ex. Butadiene) using an olefin conversion unit (OCU) and simultaneously isomerization of 1-butene into 2-butene, polymerization of polybutene, and then metathaesis of 2-butene and an ethylene into propylene. But, this method also involves production of polybutenes in the presence of a large quantity of 2-butene, which results in low mileage of catalyst.
The C4 oil produced in the contact degradation of medium-quality oil during the petroleum refining process and the C4 residue produced in the pyrolysis of naphtha contain 20 to 50 wt. % of 1-butene or 2-butene. The use of the C4 olefin in the production of polybutene may result in high halogen content and low vinylidene content in the polybutene product. Further, a high content of n-butene such as 1-butene, etc. present in the C4 olefin (i.e., feedstock) may cause deterioration in the catalytic activity, the quality of polybutene, or the productivity per unit feedstock. As a solution to this problem, a high-purity isobutene can be used. There are several methods of producing (isolating) isobutenes from the C4 mixture: (1) t-butyl alcohol (TBA) dehydration that combines the hydration reaction and the dehydration reaction; (2) methyl t-butyl ether (MTBE) cracking that includes an addition of methanol to isobutene using an acid catalyst and then a cracking into isobutene; and (3) isobutane dehydrogenation. All of these methods, however, take a high expense to produce (isolate) isobutenes, causing a rise of the polybutene cost.